1. Field of the Invention
The present invention relates to an aqueous solvent type of flux used for soldering electronic components on a circuit substrate, and a circuit substrate having a flux residue. The present invention is also concerned with a mounted or unmounted circuit substrate. In this present disclosure, the "mounted or unmounted" circuit substrate is understood to refer to a circuit substrate on which electronic components are or are not mounted.
2. Prior Art
A circuit substrate, for instance, a printed circuit substrate is fabricated by forming a circuit interconnecting pattern on a copper-clad laminate, for example. One circuit unit can then be formed by mounting electronic components on the circuit substrate. The electronic components such as capacitors and resistors are connected and fixed by soldering to a copper foil land, i.e., a soldering land of the circuit interconnecting pattern.
The soldering of electronic components on a printed circuit substrate is achieved by a so-called flow soldering process wherein a chip form of electronic component having electrodes at both its ends is provisionally held at a given position of the printed circuit substrate such that the electrodes are located at a soldering land, and the provisionally held electronic component is allowed to come in contact with a flow of molten solder, so that the electronic component can be soldered on the soldering land, or a so called reflow soldering process wherein while the electrodes of an electronic component are located at a soldering land precoated with solder paste, as mentioned just above, solder powders of the soldering paste are molten by heating, so that the electronic component can be soldered on the soldering land.
Regardless of whether the flow or the reflow soldering process is used, the electronic component is allowed to come in contact with a flow of molten solder or is coated with solder paste after a flux has been applied on the soldering land. By doing so, the oxidation of copper foils of the soldering land due to heat, etc., generated during soldering can be avoided to make soldering satisfactory. The soldering land of a printed circuit substrate is heated to 200.degree. C. to 300.degree. C. during soldering. If the soldering flux is applied on the soldering land to form flux coating thereon, then the flux film cuts off oxygen to prevent oxidation of the copper foils and reduces the oxides that have already occurred, so that the molten solder call be well wetted. This is true even when the surface of the soldering land is exposed or provided with a protecting film.
For such a flux, a rosin type of resin (hereinafter called the rosin) is often used. This flux is composed mainly of a rosin in and contains activators such as amine halides and organic acids as subordinate components, all dissolved in an alcohol solvent. For this alcohol solvent, an alcohol mixture composed mainly of isopropyl alcohol is generally used. One reason is that isopropyl alcohol is excellent to solve in the rosin and various activators. Another reason is that isopropyl alcohol, albeit having a relatively low boiling point, is higher in the permissible concentration for explosion in the air than other organic solvents, so that fire risk due to the volatilization of a solvent during flux production or flux coating can be substantially reduced. Still another reason is that isopropyl alcohol is inexpensive.
However, isopropyl alcohol is diffused in the air during flux coating, and so is a factor attributable to environmental pollution, for instance, photochemical smog. It is thus desired that the amount of isopropyl alcohol diffused in the air be reduced.
To do this, some water-soluble fluxes free from any volatile organic solvent have been developed. However, since the water-soluble substance used, for instance, is glycol ether, there are several problems. For example, if the flux is allowed to leave to be coated and soldered on the flux residue, the circuit may then be short-circuited because the resulting flux film is poor in insulating properties. To avoid this, the flux film must be washed off. This additional washing step leads to another problem, productivity drops.
In an effort to avoid this problem, refined rosin composed mainly of abietic acid is dissolved in water in the form of an ammonia salt or a salt of an amine, but the amount of the ammonia or amine used must be increased to be better solved. However, the wettability of the resultant flux residue by molten solder becomes worse and so has an adverse influence on the so-called solderability. For instance, solder is bridged across adjacent soldering lands (solder bridging), or an unsoldered area remains. Besides, the flux residue, because of containing a large amount of ammonia or the amine, is poor in electrical insulating properties and, hence, in reliability, because the circuit is often short-circuited and fails to function normally.